The present invention relates to a warning indicator of internal faults in power transformers used on energy supply systems. More specifically, the present invention relates to a device which reacts to a transient over-pressure occurring invariably as a consequence of an internal arcing fault within a power transformer.
Nearly all pole mounted distribution transformers are protected by a fused cutout which are operated by a lineman to energize particular circuits and which open in the event of an overload or internal fault. If there is an overload in the system and the fuse operates, then a lineman can easily spot the open cutout and knows that the transformer adjacent the fuse is out of service. If the fault is downstream of the transformer, then once that fault has been corrected it is a simple matter for the lineman to re-fuse the cutout to re-energize the circuit. If, however, the fault is in the transformer, then closing the cutout will produce arcing within the transformer, and in extreme cases explosion of the transformer can occur causing damages and possible injury to the lineman.
It is known that there is a transient pressure surge inside of a transformer whenever an internal arcing fault occurs. This is invariably the case because the energy dissipated in an electric arc is transferred as heat to its surroundings. The resulting rapid increase in temperature then causes the pressure rise inside the transformer. Various devices have been suggested to prevent pressure rises in transformer tanks. It is know for instance, by Hill in U.S. Pat. No. 1,709,723 that a pressure relief can be provided by a rupture of a diaphragm in the casing of the transformer. However, a diaphragm will rupture whenever the pressure exeeds a certain predetermined level, moreover a diaphragm cannot differentiate between a slow pressure buildup and a fast pressure surge. In addition, a diaphragm such as shown by Hill cannot be prevented from undesirable rupturing caused by, for example, an increase in the ambient temperature and, is therefore, not suitable for use as an indicator of internal arcing faults.
It is known that there is a transient surge in pressure inside a transformer when an internal arcing fault occurs. This is invariably the case because arcing produces a marked increase in temperature which vaporizes some of the oil within the transformer.